The present invention relates to a progressive power lens configured to have positive power in a distance portion and to have a progressive surface formed at least on an back surface (an eye side surface) thereof.
Progressive power lenses have been widely used for correction of presbyopia. In order to secure a high degree of processing flexibility and to secure a wide clear vision area, it is preferable that a progressive power lens is configured such that a progressive surface is formed on a back surface of the lens. An example of such a progressive power lens is disclosed in International publication No. WO 97/19382 (hereafter, referred to as a document 1).
It is preferable that a spectacle lens is formed to be a meniscus lens. Such a view is ascribable to the fact that if a back surface of a spectacle lens is a convex surface, problems as described below may arise. That is, if the spectacle lens is placed on a table while a back surface thereof is downwardly oriented, a central portion of the back surface contacts a top surface of the table. In this case, the central portion of the back surface of the lens may be scratched. Also, in such a case, a skin or body hairs (e.g., eyelashes) of a wearer tend to contact the convex surface, by which the convex surface may be smeared.
For this reason, the spectacle lens disclosed in the document 1 is designed in such a manner that a surface power of an object side surface (front surface) is larger than a sum of a vertex power of a distance portion and an addition power so that a meniscus shape is attained.
However, if the above mentioned design manner is adopted in designing a progressive power lens having a positive power in a distance portion thereof, a thickness of the central portion of the lens becomes large, and thereby the reduction of the lens thickness can not be attained.